U.S. patent number 10,320,261 [Application Number 15/377,117] was granted by the patent office on 2019-06-11 for rotor alignment for reducing vibrations and noise.
This patent grant is currently assigned to Siemens Aktiengesellschaft. The grantee listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Klaus Buttner, Klaus Kirchner, Matthias Warmuth, Norbert Wohner.
United States Patent |
10,320,261 |
Buttner , et al. |
June 11, 2019 |
Rotor alignment for reducing vibrations and noise
Abstract
For reducing vibrations and noise in electrical machines a rotor
is mounted in a magnetic alignment device while a bearing shield is
loosely held relative to a stator. The rotor is driven by the
stator and vibrations of the rotor are detected. A magnetic
alignment device is controlled so as to reduce the vibrations.
Finally the bearing shield is fixed to the stator in a position
determined by the controlling of the magnetic alignment device.
Thus electromagnetic forces are taken into consideration during
vibration and noise reduction.
Inventors: |
Buttner; Klaus (Hollstadt,
DE), Kirchner; Klaus (Ostheim, DE),
Warmuth; Matthias (Windshausen, DE), Wohner;
Norbert (Heustreu, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Munchen |
N/A |
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
(Munchen, DE)
|
Family
ID: |
54849831 |
Appl.
No.: |
15/377,117 |
Filed: |
December 13, 2016 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20170170703 A1 |
Jun 15, 2017 |
|
Foreign Application Priority Data
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|
|
|
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Dec 14, 2015 [EP] |
|
|
15199807 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K
5/24 (20130101); H02K 5/15 (20130101); H02K
7/09 (20130101); H02K 15/165 (20130101) |
Current International
Class: |
H02K
5/15 (20060101); H02K 5/24 (20060101); H02K
15/16 (20060101); H02K 7/09 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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104863869 |
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Aug 2015 |
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CN |
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204741363 |
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Nov 2015 |
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CN |
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19 41 558 |
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Aug 1970 |
|
DE |
|
3742149 |
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Jun 1989 |
|
DE |
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196 19 997 |
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Nov 1997 |
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DE |
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602 10 482 |
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Dec 2006 |
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DE |
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10 2010 043 042 |
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May 2012 |
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DE |
|
010752 |
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Oct 2008 |
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EA |
|
2007181325 |
|
Jul 2007 |
|
JP |
|
1115171 |
|
Sep 1984 |
|
SU |
|
Primary Examiner: Pham; Emily P
Attorney, Agent or Firm: Henry M. Feiereisen LLC
Claims
What is claimed as new and desired to be protected by Letters
Patent is set forth in the appended claims and includes equivalents
of the elements recited therein:
1. A method for mounting a rotor in a stator of an electrical
machine, comprising: mounting the rotor in a magnetic alignment
device, while a bearing shield, to which at least one rolling
bearing for support of the rotor is fastened, is loosely held in
relation to the stator; driving the rotor by the stator; detecting
first vibrations of the rotor; controlling the magnetic alignment
device such as to reduce the first vibrations; and fixing the
bearing shield to the stator in a position determined by the
controlling of the magnetic alignment device.
2. The method of claim 1, further comprising detecting second
vibrations of a housing of the electrical machine; and controlling
the magnetic alignment device also in response to the second
vibrations.
3. The method of claim 1, further comprising fixing the bearing
shield to the stator via a housing of the electrical machine.
4. The method of claim 3, further comprising: mounting a centering
portion to a front face of the housing for displacement in relation
to a remaining part of the housing perpendicular to an axis of the
rotor; and fixedly connecting the centering portion to the
remaining part of the housing as the bearing shield is fixed to the
stator.
5. The method of claim 3, further comprising: fixedly connecting a
centering portion to a front face of the housing; mounting the
bearing shield to the centering portion for displacement in a
direction perpendicular to the axis of the rotor; and fixedly
connecting the centering portion to the bearing shield as the
bearing shield is fixed to the stator.
6. The method of claim 1, further comprising limiting a radial
displacement of the rotor relative to the stator to a predetermined
maximum value during the controlling of the magnetic alignment
device.
7. The method of claim 3, further comprising limiting a radial
displacement of the rotor relative to the stator by a predetermined
maximum clearance of the bearing shield relative to the
housing.
8. A mounting system for mounting a rotor in a stator of an
electrical machine, comprising: a magnetic alignment device for
mounting the rotor in a rolling bearing; an activation device
configured to activate the rotor by the stator; a sensor device
configured to detect vibrations of the rotor in an operating state
in which the rotor is driven by the stator; and a control device
configured to control the magnetic alignment device such as to
reduce the vibrations.
9. The mounting system of claim 8, wherein the magnetic alignment
device includes a first magnetic bearing device and a second
magnetic bearing device, the first magnetic bearing device being
mounted radially with one end of a shaft of the rotor, and the
second magnetic bearing device being mounted radially with another
end of the shaft of the rotor.
10. The mounting system of claim 9, wherein the first magnetic
bearing device includes a magnetic axial bearing.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of European Patent
Application, Serial No. 15199807.7, filed Dec. 14, 2015, pursuant
to 35 U.S.C. 119(a)-(d), the content of which is incorporated
herein by reference in its entirety as if fully set forth
herein.
BACKGROUND OF THE INVENTION
The invention relates to a method for mounting a rotor in a stator
of an electrical machine, and to a mounting system for mounting
such a rotor in a stator.
The following discussion of related art is provided to assist the
reader in understanding the advantages of the invention, and is not
to be construed as an admission that this related art is prior art
to this invention.
Electric motors, in particular, in synchronous-reluctance
technology, are frequently subjected to vibrations and noise during
operation due to asymmetries. Such vibrations and noise are
generally disruptive for the intended use, but also for the
operating personnel. Therefore, attempts have been made to reduce
such vibrations and noise. However, not only electric motors, but
in some circumstances, generators are also affected by asymmetries.
Here, attempts have also been made to minimize corresponding
vibrations and noise, if required.
The causes of these asymmetries firstly result from manufacturing
inaccuracies. These include, for example, shape tolerances and
positional tolerances of the stator laminations and rotor
laminations. Further, inaccuracies may result generally from
components such as the housing, stator, bearing shield, bearings
and rotor of the electrical machine. In some circumstances, the
inaccuracies accumulate, when the rotor is mounted in the
stator.
Centering devices are generally used, when mounting a rotor in a
stator. A radial fixing of the rotor relative to the stator is
carried out thereby. The two components, housing and bearing
shields, in this case generally have interference fits on the
centering edges. This results in only one fixed position of the
rotor relative to the stator.
Asymmetries may also result from the material itself. For example,
the magnetic properties of magnetic steel sheet depend on the
rolling direction. Significant force-related asymmetries may
result, solely, from these minimal magnetic differences during
operation of the electrical machine.
The material properties are typically not considered in an ideal
alignment of the rotor axis. Motors in synchronous-reluctance
technology are generally constructed with a smaller air gap than
asynchronous motors. This is because, with a smaller air gap, there
is a greater differentiation between the d-axis and q-axis, and
also, the efficiency may be increased. At the same time, however,
the susceptibility to vibrations, which originate in the
electromagnetic system is increased.
It would therefore be desirable and advantageous to address prior
art shortcomings and to provide improved rotor alignment in a
stator by being able to consider asymmetrical magnetic forces.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a method for mounting a
rotor in a stator of an electrical machine includes mounting the
rotor in a magnetic alignment device, while a bearing shield, to
which at least one rolling bearing for support of the rotor is
fastened, is loosely held in relation to the stator, driving the
rotor by the stator, detecting first vibrations of the rotor,
controlling the magnetic alignment device such as to reduce the
first vibrations, and fixing the bearing shield to the stator in a
position determined by the controlling of the magnetic alignment
device.
Accordingly, an electrical machine is mounted, in which, in
subsequent operation, the rotor is mounted by a rolling bearing
in/on the stator. The rolling bearing itself is fastened to or in a
bearing shield. At the end of the mounting procedure, the bearing
shield is then intended to be fixedly fastened to the stator,
indirectly, or directly. However, in order to find the correct
position, the rotor is initially mounted in a magnetic alignment
device, while the bearing shield is still loosely held relative to
the stator. Thus, the bearing shield, together with the rolling
bearing and the rotor, are radially displaceable. For the
alignment, the rotor is now set in rotation, i.e. by the stator,
which also exerts corresponding forces on the rotor during
operation. Thus, realistic forces, which occur during the operation
of the rotor are now present. Vibrations of the rotor related
thereto (for example in the form of forces or deflections over
time), which result from asymmetrical shapes or effects of force
are detected. In response thereto, the magnetic alignment device is
controlled so as to reduce the vibrations. The vibrations can even
be minimized or eliminated by appropriately controlling the
magnetic alignment device. In this state, when the vibrations are
reduced, minimized or eliminated, the bearing shield is now
fastened to the stator indirectly or directly. Thus, an alignment
of the rotor in the stator can be implemented while vibrations and
also noise are reduced. Suitably, the rotor is driven during
alignment as if driven during normal operation, in particular in
terms of the rotational speed.
According to another advantageous feature of the invention, second
vibrations of a housing of the electrical machine can be detected,
and the magnetic alignment device can be controlled also in
response to the second vibrations. Therefore, in addition to the
first vibrations, which are detected relative to the rotor, also
vibrations relative to the housing are included in the controlling
procedure. Similarly, for the controlling procedure, other
components of the electrical machine may also be taken into account
during the controlling, such as, for example the stator or
amounting foot relative to its vibrations.
According to another advantageous feature of the invention, the
bearing shield can be fixed to the stator via a housing of the
electrical machine. The electrical machine thus has a housing to
which the bearing shield is fastened. The housing, in turn, is
typically rigidly connected to the stator. As a result, the bearing
shield is indirectly fixed to the stator.
According to another advantageous feature of the invention, a
centering portion can be mounted to a front face of the housing for
displacement in relation to a remaining part of the housing
perpendicular to an axis of the rotor, and the centering portion
can be fixedly connected to the remaining part of the housing as
the bearing shield is fixed to the stator. As a result, an annular
centering portion may, for example, be displaceably arranged on the
front face of a housing and can then be fixed to the housing, for
example by pins, adhesive or by other fastening procedures.
According to another advantageous feature of the invention, a
centering portion can be fixedly connected to the front face of the
housing, the bearing shield can then be mounted to the centering
portion for displacement in a direction perpendicular to the axis
of the rotor, and the centering portion can be fixedly connected to
the bearing shield as the bearing shield is fixed to the stator. In
this case, the centering portion is therefore not displaceable
relative to the housing, but the bearing shield is displaceable
relative to the centering portion. This may be implemented, for
example, by the centering portion having an optionally limited
surface on which the bearing shield is movable.
According to another advantageous feature of the invention, a
radial displacement of the rotor relative to the stator can be
limited to a predetermined maximum value during the controlling of
the magnetic alignment device. This is the case, for example, when
the axial displacement of the bearing shield or the centering
portion is limited by corresponding shoulders on the housing and/or
on the centering portion. As a result, the rotor is prevented, for
example, from moving toward the stator, when the rotor is aligned
in the stator. Thus, it is particularly advantageous, when the
rotor is displaceable by less than a predetermined nominal air gap
between the rotor and stator in a radial direction.
According to another advantageous feature of the invention, a
radial displacement of the rotor relative to the stator can be
limited by a predetermined maximum clearance of the bearing shield
relative to the housing.
The limit to the displacement in the radial direction may be
implemented by a controlling procedure, but also, by a suitable
hardware-type embodiment of the components. In particular, the
components may have suitable contours, which permit such a maximum
clearance, where the rolling bearing is fastened to the housing by
the components. As a result, even in the case of an error in the
controlling procedure, it is ensured, that the rotor does not come
into contact with the stator.
According to another aspect of the invention, a mounting system for
mounting a rotor in a stator of an electrical machine includes a
magnetic alignment device for mounting the rotor in a rolling
bearing, an activation device configured to activate the rotor by
the stator, a sensor device configured to detect vibrations of the
rotor in an operating state in which the rotor is driven by the
stator, and a control device configured to control the magnetic
alignment device such as to reduce the vibrations.
In accordance with the present invention, the mounting system can
thus have an activation device in addition to the magnetic
alignment device. The electrical machine is activated by the
activation device for rotating the rotor. Suitably, the activation
device is capable to produce such activation signals, as are
applied thereto, during normal operation of the electrical machine.
A variable activation device is particularly advantageous, by which
a plurality of activation signals can be produced for very
different types of electrical machine. To realize an exact
alignment, the mounting system includes a sensor device by which
the vibrations of the rotor of the driven electrical machine can be
detected. Advantageously, the sensor device is fixedly connected to
the magnetic alignment device. Finally, the mounting system
includes a controlling device for controlling the alignment device,
with the sensor signal of the sensor device for a control circuit
being provided for reducing the vibrations. Therefore, the
alignment device (for example an electromagnet), the sensor device
(for example a vibration sensor) and the control device (for
example a controller), and optionally, a power amplifier, are
located in an exemplary control circuit.
According to an advantageous feature of the invention, the
alignment device may include a first magnetic bearing device and a
second magnetic bearing device, the first magnetic bearing device
being mounted radially with one end of a shaft of the rotor, and
the second magnetic bearing device being mounted radially with
another end of the shaft of the rotor. The shaft of the rotor may
therefore be mounted between the two magnetic bearing devices. In
this manner, the rotor of the electrical machine can be radially
aligned solely by magnetic forces.
According to an advantageous feature of the invention, the first
magnetic bearing device can include a magnetic axial bearing. This
ensures that the rotor of the electrical machine, when exclusively
mounted by the two magnetic bearing devices, is also stabilized in
an axial direction.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention will be more
readily apparent upon reading the following description of
currently preferred exemplified embodiments of the invention with
reference to the accompanying drawings, in which:
FIG. 1 is a schematic view for representing a radial force
distribution in an electrical machine including a rotor and
stator;
FIG. 2 is a schematic view of the electrical machine of FIG. 1 in a
balanced state in terms of force;
FIG. 3 is a sectional view of an exemplary embodiment of a mounting
system according to the invention with incorporated electrical
machine; and
FIG. 4 is a basic sketch of a circuit diagram of a control circuit
of a mounting system according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Throughout the figures, same or corresponding elements may
generally be indicated by same reference numerals. These depicted
embodiments are to be understood as illustrative of the invention
and not as limiting In any way. It should also be understood that
the figures are not necessarily to scale and that the embodiments
are sometimes illustrated by graphic symbols, phantom lines,
diagrammatic representations and fragmentary views. In certain
instances, details which are not necessary for an understanding of
the present invention or which render other details difficult to
perceive may have been omitted.
The exemplary embodiments described in more detail hereinafter
represent preferred embodiments of the present invention. It should
be noted that the individual features may be implemented not only
in the combinations set forth but also in other technically
expedient combinations or separately.
Turning now to the drawing to FIG. 1, there is shown schematically
a force distribution in an electrical machine, which includes a
stator 1 and a rotor 2. For the sake of simplicity, in this case,
the force distribution is only shown in an x-y plane, which extends
perpendicular to the axis of the rotor 2. In principle, naturally
forces may also be produced in the z-direction, i.e. perpendicular
to the x-y plane. These forces, however, are not considered here in
a simplified embodiment.
For the sake of manufacturing accuracy, the position of the rotor 2
is determined according to a tolerance chain. The air gap may
therefore, fluctuate considerably according to the size of motor.
Typically, the air gap in a machine with an axial height of 180 mm
may fluctuate up to 0.25 mm. Hitherto, the object was to bring the
position of the rotor axis 3, resulting from the manufacturing
dimensions, as close as possible, to the ideal geometric rotor axis
4, by the greatest possible manufacturing accuracy. This is
associated with a high cost, and depending on how the tolerances
coincide, is still subject to a high degree of inaccuracy. In
addition, not all affected geometries are able to be considered,
such as, for example the bottom of the groove of the stator core.
Moreover, the material properties, which become effective inside
the magnetic circuit are not considered. These material-specific
asymmetries, however, have a significant effect on the excitation.
This results, for example, in the three force vectors F1, F2, F3
having variable values starting from the geometric axis 3 and
uniformly distributed in the peripheral direction. Therefore, it is
the object to reduce, as far as possible, the asymmetries, which
result from different forces, in order to reduce additionally the
resulting vibrations.
To this end, according to FIG. 2, when mounting the rotor, the
alignment does not take place according to the resulting
manufacturing rotor axis 3 or the ideal geometric rotor axis 4, but
also according to an electromagnetically acting rotor axis 5. In
this case, when mounting the rotor, the electric machine, for
example the motor, is electrically operated, for example by an
activation device. The mechanical rotor bearings are not yet
fixedly connected to the stator 1. In a practical example,
therefore, the bearing shields are not yet fixedly mounted on the
housing. By an alignment device, in which, the rotor of the
electrical machine is mounted for the mounting, the rotor is
aligned in a way, that the forces acting on the rotor cancel one
another out, as far as possible. This is the case, for example,
when as in FIG. 2, the force vectors F1, F2, F3, which are
distributed uniformly over the circumference, and which act on the
rotor 2, are of the same length. During the operation of the rotor,
therefore, in some circumstances, it is possible to carry out the
alignment of the axis of the rotor 2 according to a so-called
"electromagnetically acting rotor axis" 5. During such an
alignment, this results in a symmetry of the forces, and thus, a
reduction in the vibrations, which is as complete as possible for
the predetermined operating mode.
FIG. 3 shows an exemplary mounting system, an electrical machine,
for example a motor 6, being mounted therein. The mounting system,
in this case, has two magnetic bearing devices 7 and 8, a shaft 9
of the rotor 2 being mounted therebetween.
In the selected example, the electrical machine and/or the motor 6
has a housing 10, which is fixedly connected to the stator 1. On
the front face, the housing 10 has on both sides, one respective
centering portion 11, which is of annular configuration. The
centering portion 11 has a shoulder 12, by which, it is able to be
connected centrally to the remaining housing, in this case, a
cylinder casing-shaped housing portion. One respective bearing
shield 13, for example, is fastened by pins to each centering
portion 11. A bearing 14, which bears the shaft 9, is in turn,
fastened to each bearing shield.
Before mounting, for example, according to one embodiment, the
bearing shield 13 is not fixedly connected to the centering portion
12. This means that the rotor 2 is not fixedly connected to the
stator 1. Instead, the air gap 15 is thus variable.
In the example of FIG. 3, the mounting system includes the two
magnetic bearing devices 7 and 8. In a simplified embodiment,
however, the mounting system might also have only one magnetic
bearing device, and the electrical machine would be initially
aligned on one side for the mounting and then reversed and aligned
on the other side.
The magnetic bearing device 7, shown to the left in the example of
FIG. 3, has a rotor 16, which serves as a receiver for the rotor
shaft 9 of the electrical machine and/or of the motor 6. For
example, to this end, an axial force F is exerted by the rotor 16
onto the shaft 9. An axial magnetic bearing 17 is suitable, and a
disk is thereby fastened directly to the rotor 16. An electromagnet
of the axial bearing 17 may, for example, push the disk in the
z-direction onto the front face of the shaft 9.
The magnetic bearing device 7 also has a magnetic radial bearing
18, where the rotor 16 is mounted thereby radially. The radial
magnetic bearing 18 is, for example, activated by an activation
device, so that the rotor 16 together with the shaft 9 during
operation of the electrical machine, i.e. during the rotation of
the rotor 2, is floatingly mounted. A sensor 19 of the magnetic
bearing device 7, in this case, may record vibrations of the rotor
16 of the magnetic bearing device 7, and thus of the rotor 2 of the
electrical machine.
The magnetic bearing device 8 is constructed in a similar manner on
the right-hand side of the mounting system. It has a rotor 20,
which is mounted in a magnetic radial bearing 21. In this case, a
variable axial bearing is provided. In the z-direction, i.e. in the
axial direction, the rotor is axially fixed by a corresponding
support 22. Moreover, the second magnetic bearing device 8 also has
a sensor 23 for detecting vibrations of the rotor 20.
The two magnetic bearing devices 7 and 8 are fixed relative to one
another, for example, by a mounting 24. This mounting 24 is also
able to ensure that the z-axis is fixed, which the arrows 25
indicate. Moreover, a stop 26 may be provided for the axial
positioning of the housing 10 of the electrical machine, if it is
not sufficiently supported on the mounting 24.
In order to record, not only vibrations of the shaft 9 of the rotor
2, but also those of the housing 10, for example, further vibration
sensors may be provided on the electrical machine. For example, one
or more vibration sensors 27 are arranged on the housing 10, in
order to detect directly, any vibrations of the housing 10.
Before aligning the rotor 2 in the stator 1, therefore, the rotor 2
is not yet fixedly connected to the stator 1. For example, the
bearing shield 13 is not yet fixed to the centering device 11
and/or the centering device 11 is not yet fixed to the remaining
housing 10. In this state, the rotor 2 is only mounted by the
alignment device (in this case the two magnetic bearing devices 7
and 8). For the mounting, the rotor is now operated electrically,
and advantageously, as it is also operated in subsequent normal
operation. Thus, it is ensured, that as few vibrations as possible,
occur during normal operation. The electrical machine is thus
activated, for example, at a specific frequency or with a specific
signal shape. On the two motor bearings 7 and 8, advantageously,
only the radially aligned forces of variable size are optionally
detected in all directions (360 degrees). By the alignment device
(in this case the magnetic bearing devices 7 and 8), depending on
the measured forces, the rotor is displaced in the x-direction and
y-direction until the forces acting in all directions are virtually
the same size. In a specific embodiment, the vibrations may be
detected directly or indirectly by force measurements.
In order to permit a displacement of the rotor axis, therefore,
centering devices should not be provided on the bearing shield, or
the centering devices are provided on the bearing shield with a
degree of freedom in the x-direction and y-direction. As in the
example of FIG. 3, the centering portion 11 relative to the bearing
shield 13 may represent a planar surface on which the bearing
shield is displaceable. The centering portion 11 is in this case,
only centered relative to the remaining housing. The possibility
for displacement, however, may be provided both on the bearing
shield relative to the housing and on a separate hub component (in
FIG. 3 the centering portion 11) on the bearing shield 13.
Advantageously, the extent of the degree of freedom for the
displacement of the rotor axis is fixed depending on the desired
air gap. Thus, the maximum permitted radial displacement may
correspond to a difference between the air gap dimension, minus 0.2
mm. Advantageously, the degree of freedom of the displacement in
the centering edge (see shoulders 12) is also less than the air gap
of the alignment unit (magnetic bearing). Thus, the rotor may be
set in rotation without contact taking place in the alignment
unit.
For the alignment, the alignment device is controlled by a control
device. A corresponding control circuit is shown symbolically in
FIG. 4. The alignment device has an electromagnet 28 which acts on
the rotor 2. Accordingly, for example, the x-position of the rotor
2 is altered, indicated in FIG. 4 by the distance x.sub.s. The
electromagnet 28 is, for example, part of the magnetic radial
bearing 18. The vibration sensor 19, or alternatively a sensor for
the force measurement, records vibrations or forces of the rotor 2
and delivers a corresponding measurement signal to a controller 29.
This controller delivers a corresponding control variable, for
example via a power amplifier 30, to the electromagnet 28. By such
a feedback, an alignment of the rotor is possible, so that
vibrations and noise are reduced, and optionally, may even be
eliminated. In order to take into account the properties of the
housing in the system, the vibration sensors 27 on the housing 10
may be additionally incorporated in the control circuit. Thus, a
fine adjustment of the rotor 2 might be possible.
At the end of the alignment procedure, therefore, when the ideal
electromagnetic position of the rotor has been established, the
bearing shields 13 and/or the centering portions 11 may be fixed to
the housing 10 and/or stator 1. Thus, the rotor 2 is in a fixed
local position relative to the stator 1. Vibrations and noise of
the electrical machine are thus reduced in the designated normal
operation.
Advantageously, a smaller air gap may therefore be provided between
the rotor and stator, resulting in an increase in efficiency.
Moreover, motors may be produced with reduced noise, in particular,
in the relatively sensitive reluctance technology field.
While the invention has been illustrated and described in
connection with currently preferred embodiments shown and described
in detail, it is not intended to be limited to the details shown
since various modifications and structural changes may be made
without departing in any way from the spirit and scope of the
present invention. The embodiments were chosen and described in
order to explain the principles of the invention and practical
application to thereby enable a person skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated.
* * * * *